Suspending a result set and continuing from a suspended result set for transparent session migration

ABSTRACT

A system, method, computer program and system for suspending a result set and continuing from a suspended result set for transparent session migration is disclosed. The rows from a row source are suspended into storage, so that after session migration, the client fetches can transparently continue from the suspended result set.

CROSS REFERENCED AND RELATED APPLICATIONS

This application claims benefit of Provisional Application Ser. No.60/601,346; filed on Aug. 12, 2004, which is hereby incorporated byreference as if fully set forth herein.

This application is related to the following other applications: (a)Ser. No. 10/144,665, filed May 10, 2002, entitled “Method and System forScrollable Cursors”; (b) Ser. No. 10/144,666, filed May 10, 2002,entitled “Method and Mechanism for Reducing Client-Side Memory Footprintof Data for Scrollable Cursors”; (c) Ser. No. 10/917,953 filed Aug. 12,2004, entitled “Transparent Session Migration Across Servers”, (d) Ser.No. 11/097,065 filed on even date herewith entitled “Suspending a ResultSet and Continuing from a Suspended Result Set for Scrollable Cursors”,based on Provisional Application Ser. No. 60/601,259; (e) Ser. No.11/097,422 filed on even date herewith entitled “Suspending a Result Setand Continuing from a Suspended Result Set”, based on ProvisionalApplication Ser. No. 60/601,250; and (f) U.S. Provisional ApplicationSerial No. 60/495,368 filed Aug. 14, 2003, entitled “computer ResourceProvisioning”. Each of the above disclosures is hereby incorporated byreference as if fully set forth herein.

BACKGROUND

The invention is related to computer systems, and more particularly toquery processing.

Many computer systems utilize servers, such as “database servers” tostore and maintain information. In a client-server computer system model(or a multi-tiered computer architecture), users that wish to access ormodify information at the server are often located at a “client.” Tofacilitate the explanation the terms “database server” and “databaseclient” may be used in this document in place of “server” and “client;”however, the embodiments are not limited in applicability to databasesystems, and indeed, can be utilized in many other types of computersystems.

In client-server systems, users at a database client submit commands tothe database server to store, modify, or retrieve data. In response tothe user commands, data manipulation or query activities are performedat the database server, with data results returned back to the databaseclient. In networked environments, the database server often performsdata manipulation or query commands submitted by remotely locatedclients. The client may establish a direct connection to the databaseserver over the network, or may establish a connection through one ormore intervening system components, such as an application server ortransaction processing monitor. In either case, the database serverprocesses the user commands and generates appropriate data outputs to bereturned to the client.

A common database function, for example, is to perform data queriesusing a database query language such as SQL. The database serverreceives each query and generates a query result set that satisfies thecriteria defined by a particular query. The query result set issubsequently transferred to the database client from which the queryoriginated.

A cursor is a handle to a query execution area, e.g., an area in memoryin which a parsed structured query language (“SQL”) statement and otherinformation for processing the SQL statement is stored. In many databasesystems, a cursor is an available resource for accessing or parsing SQLstatements embedded within an application. Cursors enable a SQLstatement to retrieve rows from a database table into a given resultset, which can then be accessed, displayed, or operated upon.

Each cursor has a client side cursor state and a server side cursorstate. In server processing there are circumstances when the server sidecursor state will become unavailable. For example, when performing loadbalancing a server will migrate, restart, and/or redirect a session(e.g., a client and/or server session) from one server to another. Thiscan cause an interruption in result set availability. It may benecessary to have access to a result set for the life of a client sidecursor.

SUMMARY

The embodiments described herein are directed to a method, system, andcomputer program product for suspending a result set and continuing froma suspended result set for session migration. The rows from a row sourceare suspended into storage so that when a fetch is requested the datacan be sent (continued) from storage and does not depend on the originalrow source or session to be available. In embodiment(s), suspending aresult set is used to migrate a server session. A method for suspendinga result set for transparently migrating a session includes receivingnotification that the first session is selected for migration, preparingto migrate by suspending the result set for the first session,determining that the first session is ready to migrate, and migratingthe first session.

Further details of the embodiments are described below in the detaileddescription, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a flowchart 150 of an example of a generic server processinga client fetch query.

FIG. 1B is a flowchart 100, a generic example of a server processing aclient fetch of a result set with an option of suspending the resultset.

FIG. 2A is a representation of a process 200, an example implementationof suspending a row of a server result set.

FIG. 2B is a representation of process 240, a generic example of aserver suspending a server row source.

FIG. 2C is a representation of a row mapping index.

FIG. 2D is a representation of process 270, the suspend all cursorsprocess.

FIG. 3 is a representation of process 300, an example of fetching a rowof a result set while the result set is being suspended.

FIG. 4A is a representation of a generic data flow for suspending andfetching operations.

FIG. 4B is a representation of the data flow in one exampleimplementation of suspending and fetching operations.

FIG. 5A is a representation process 520, a generic example of continuingclient fetches from a suspended result set after suspension.

FIG. 5B is a representation of process 500, an example embodiment ofcontinuing client fetches from a suspended result set after suspension.

FIG. 6A is a flow chart of process 650, an example embodiment oftransparently migrating a session utilizing suspending a result set andcontinuing from a suspended result set.

FIG. 6B is a flow chart of process 600, an example of transparentlymigrating a session.

FIG. 7 is a graphical representation of server and client sessionsduring the suspending and continuing of a suspended result set forsession migration.

FIG. 8 is a representation of a system on which suspending a result setand continuing a suspended result set would take place.

DETAILED DESCRIPTION OF INVENTION

A method and system of suspending a result set, and continuing from asuspended result set for transparent session migration is described. Inthe following description, for the purpose of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the presented embodiments. It will be apparent,however, to one skilled in the art that the embodiments may be practicedwithout these specific details.

Result Set

The result set is the output of executing a query. The embodiments maybe applied to any type of data in the result set, including relationaldata. To facilitate the present explanation, the following descriptionis made in the context of relational data. It is noted, however, thatthe embodiments may be applied to non-relational data.

Typically, a defined sequence of actions is performed to execute a querystatement, e.g., a SQL query statement. The SQL query is typicallyprocessed using a parse, execute, fetch, and close cycle of operation. Acursor, or query execution area, is created, either in conjunction withthe SQL query or independently. The cursor may be automatically handledby the database system, or manually controlled by a programmer throughan application programming interface; The SQL statement is thereafterparsed, optimized, and executed to produce a set of rows called theresult set or row source. The rows in the result set can be extractedfrom the cursor (“fetched”) either a row at a time or in groups. A rowsource can be located in the SQL execution cursor area on the server fora database application.

For example, process 150 shown in FIG. 1A represents one way for aserver to handle fetch operations. The server waits for a client call inprocess action (152). Process action (154) processes a fetch request fora client. A row from the result set produced by the cursor is sent fromthe cursor to the client in process action (156). Process action (158)determines if there are more rows to be fetched. If there are more rows,process 150 returns to process action (156) to return another row. Ifthere are no more rows process action (160) determines if the cursorshould be closed. If the cursor should be closed, the cursor is closedin process action (162). After the cursor is closed or if the cursorshould not be closed, process 150 returns to process action (152) andwaits for the next call.

The database server produces a result set composed of a quantity of datathat can be sent to a database client. The user may initially place anexplicit request to transmit a first portion of that result set from thedatabase server to the client. At a later time, the client may request asecond portion of the result set. This process may proceed until theentire result set is sent, or until the client stops requestingadditional portions of the result set. This process results in one ormore roundtrips between the database server and the client.

A portion of the result set can be transmitted prior to the clientrequest for that portion. Transmitting information prior to it beingimmediately requested by the client is referred to as pre-fetching.Pre-fetching can be based on predictive selection of data anticipated tobe requested by a client in the future. A determination is made ofwhether there exists any need or advantage to pre-fetch. Thedetermination is made in a predictive manner based upon differentfactors. Some example factors include, but are not limited to, datacompression ratios, likelihood of the client requesting the data, androundtrip delay.

While the cursor is active it has a client side cursor state and aserver side cursor state. The cursor is used to access the result set.In certain instances the server side cursor is eliminated, disruptingaccess to the result set. However, it may be necessary to have access toa result set for the life of a client side cursor. Not having accesscould be a correctness issue. One option is to reissue the query on anew server session. However, even having to reissue the client query toobtain the next set of rows can be a correctness issue. For examplere-execution of a query may not guarantee the same results or the sameorder of results. Access can be maintained to the result set bysuspending the result set, i.e. storing the result set such that accessis preserved even in the event that the cursor is unavailable. Process240 in FIG. 2B shows an example suspend process. This process isexplained in the SUSPENDING section.

Storage

As mentioned above, when the cursor is active, operations can beperformed against the specific row or group of rows at which the cursoris pointing. In some cases, the result set can only be accessed by thespecific cursor (e.g., a specific client cursor), and only while thecursor is active. It may also be desirous, in one embodiment, to accessand operate on the result set after the server side cursor has beenclosed. Another embodiment accesses the result set from another clientsession.

One method of accommodating the various embodiments, is to suspend theresult set into the client. However, the client may not have a largeenough storage to store the entire result set or have the desire to usethe storage for the result set. Further, a client storage may only beavailable to that client. The solution to make a result set availableafter a cursor is closed and not consume much of the client storagespace, as presented in one embodiment, is to pre-fetch and store thecursor result set.

Storage for a suspended result set can take many forms. Example storageforms include but are not limited to: server cache, server workspace,server persistent storage, server memory shared across nodes, serverpersistent storage shared across nodes, session cache, session sharedmemory, session private memory, client cache, client persistent storage,other client memory, other server memory, persistent storage that doesnot incur transactional overhead such as redo or undo logging orminimizes the use of such transactional logging, or a combination of anyof the foregoing. For example, some embodiments depicted in FIG. 2-5show the suspended result set using a combination of persistent andmemory storage. The type of storage used to suspend the result set isselected by the application, or chosen internally, and depends on theintended use of the result set and availability of system resources. Theresult set storage, in one embodiment, is contiguous space. Anotherembodiment uses non-contiguous space. In one embodiment the type ofstorage may be automatically determined based on statistics collectedover a period of time. For example, suppose an application was using aresult set located on a server with limited available memory. If thisresult set is suspended, the storage location(s) chosen would bepersistent storage or a combination including persistent storage so asminimize consumption of additional memory. However, if the currentserver memory usage is low, the storage location(s) chosen for asuspended result set could be server memory.

If persistent storage is chosen as the form of storage, one embodimentuses Large Objects (LOB) for the persistent storage, however any storagethat is accessible between database nodes is acceptable. The followingare two example approaches to creating an LOB. A temporary, ornon-sharable, LOB can be created that is visible only in the databasenode in which it is created. A permanent, or sharable, LOB can becreated that is more persistent. That is, the sharable LOB is able toexist for the lifetime of a database and is sharable across databasenodes. Hence, a non-sharable LOB can be created in one database nodethat is not valid in another database node, and a sharable LOB can becreated in one database node that is valid in another database node. Thepersistent storage can be located on the server or on the client.

In one embodiment, the suspended result set storage can be in the memoryof a first node, and transmitted to a second node that is continuing thefetches. This may minimize costs (e.g., I/O costs) of accessingpersistent storage. Example of methods that may be employed toaccomplish this include distributed SQL, inter-process communications,or fast interconnects.

Packing

The suspended result set, in one embodiment, is self-describing storage.That is, the storage has a specific layout and includes informationdescribing that layout. Self-describing storage can include datadescriptions, or metadata, within the storage so that entities accessingthe storage know what to expect upon reading the storage.Self-describing storage can be helpful, for example, when theserver-side cursor is eliminated and the result set still needs to beaccessible.

The metadata of the suspended result set can be packed using one packinglogic and the values of the suspended result set can be packed usinganother packing logic. Another embodiment uses compression techniques inaddition to the packing layout. In another embodiment, the packinglayout includes a version to allow compatibility with older and futureversions. Another embodiment uses both version and compressiontechniques. Further details regarding compression techniques that can beapplied are disclosed in application Ser. No. 10/144,666, filed May 10,2002, entitled “Method and Mechanism for Reducing Client-Side MemoryFootprint of Data for Scrollable Cursors,” and/or application Ser. No.09/515,674, filed Feb. 29, 2000, entitled “Method and Mechanism forReducing Client-Side Memory Footprint of Transmitted Data,” which arehereby incorporated by reference in their entirety.

The area of the suspended result set containing the metadata is calledthe header. Table 1 shows one embodiment of the metadata content. Thiscontent can include header size, storage version, header checksum, andnumber of columns. The header size details the number of bytes themetadata consumes. The storage version specifies how to pack and unpackthe result set. The header checksum is the checksum of all the headerbytes. The number of columns details the number of columns that theresult set contains.

In one example embodiment, as shown in Table 1, the header area relatedto individual column metadata contains two metadata values: data typeand max length. The metadata for each column follows one after theother. The data type details what data type the result set columncontains. The maximum length is the maximum length that the data typewill consume and can be used to allocated the space for the receivingthe row from the suspended result set.

TABLE 1 Example Suspended result set storage header (metadata) layoutMetadata Content Header size storage version Header checksum Number ofcolumns Column 1: data type Column 1: max length Column n: data typeColumn n: max length . . .

The data types can be simple scalar types, such as a NUMBER or CHAR, orcomplex non-scalar, such as a collector type or object type. In oneembodiment, such data types are packed as a linear image of the originalobject and the header can include additional type metadata information,such as for example, object (or collector) ID length and object (orcollector) ID bytes. In one embodiment this additional metadatainformation can be used by the client to unpack the stored linear imageinto its original collector type data form.

One embodiment for the row packing format is shown in Table 2. The rowpacking format can include: row size, and column specific informationfor each column such as actual length, column flags and column value.The row size can be used as a checksum to ensure all the bytes werepacked and unpacked correctly. In another embodiment, the row size andcolumn size can be stored with the row mapping index (see FIG. 2C andrelated text), as opposed to being stored in the suspended result set.Note that data in the suspended result set can be packed using actuallength to minimize storage size. Another embodiment combines the rowpacking layout with data compression techniques to save space whilesuspending and continuing from a suspended result set.

TABLE 2 Example Suspended Result Set Packed Row (data) format RowContent Rowsize Column 1: actual length Column 1: flags Column 1: valueColumn n: . . .

When persistent storage is used, one embodiment organizes the writes andreads into a particular size called a chunk. The actual size of thecolumn value can vary so the readwrite sizes can be converted intochunks at run-time. The conversion to chunks is aided by the layout orpacking logic. The packing logic provides a map so that flags can act asplace holders and identify suspended actions. Memory chunks are sized tooptimize persistent storage access instead of accessing persistentstorage columns or rows at a time.

The memory buffer could be exhausted at any point of writing thesebytes. The state is maintained so that next call (and next chunk) cancontinue from where the packing-phase was last suspended. For example,assume only a portion of the bytes of the size field (at a particularcolumn) could be written when the end of the chunk was reached. Thestate and offset information is stored. In the next call, the savedstate and offset value are used to write the next byte(s).

Suspending

The suspending operation captures rows of the row source into storage sothat if something were to happen to the row source, to the servercursor, or to the session, the result set would still be available. Thisallows the result set to be available for the life of the client cursor,not just the life of the server cursor. The suspending operation canhandle distributed SQL queries and queries involving clientpre-fetching, or both. In addition, suspending during an already runningfetch does not require any application changes. The suspension operationoccurs transparently with minimal impact to client fetch requests. Thatis, the suspension operation is performed in a manner that minimizesclient fetch request delay so that the client remains unaware of thesuspension. In one embodiment, the suspending operation of a cursorresult set can be completely independent from fetch operations and/orother continuing-from-suspended-result-set operations that may beperformed upon that result set. For example, in one embodiment, thesuspending can occur after every round-trip from the client even thoughthe client request may be for a DML or for a different cursor. In oneembodiment suspending of the result set on the cursor that has beenparsed and executed on the server can occur before any rows are sent tothe client. In another embodiment, suspending can occur after rows aresent to the client, i.e., in the middle of fetching. Suspending allowsthe pending result set to be retained without retaining the servercursor area or the server session. In one example, the pending resultset for sequential cursors would be the rows not yet seen by the client.In another example, the pending result set for scrollable cursors canpotentially be the entire result set.

One way to suspend a result set is by process 240, a generic example ofa server suspending a row source, shown in FIG. 2B. Process 240 is areentrant process, i.e., it can be stopped and invoked later to suspendadditional rows. The following description of FIG. 2B will referenceFIG. 4B. Process 240 starts at process action (242) when a call is madeto suspend a server result set, e.g., by another process. Process action(244) determines if there are more rows in the row source 422. The rowsource is the SQL execution cursor area on the server for a databaseapplication. If there are more rows in the row source 422, the row issuspended in process action (246) by storing the row in the suspendedresult set storage 424. Then client 426 can access the suspended resultset in storage 424 even after the cursor has become unavailable.

Process action (248) determines if any of a set of constraints have beenviolated. Constraints may be based upon any factors/parameters that canbe set to control whether, how, and when, the present suspending processcan be employed, e.g., the constraint conditions may be related to timeand space and set to minimize the impact that the suspending process 200has on other server operations (e.g., client fetches). For example, theconstraint may include or relate to factors that prevent the processfrom affecting client latency or system performance. This may relate to,for example, server resource utilization, time-based factors, number ofrows, consumption of I/O resources, etc. The constraints may be basedupon, for example, each cursor, all cursors of a session, all sessionsin a node, and/or all nodes of a cluster. The suspend process willsuspend as many rows in the result set row source (e.g., action 244 ofFIG. 2B) until it is limited by one or more (or even all) of theconstraints.

In one embodiment, the timeout constraint is set by padding the averagecursor roundtrip time with an absolute delay value or a percent delayfactor. This prevents delay of any roundtrip that uses the whole(average roundtrip time+delay value) time. It also allows the suspendoperation to occur, and adds only a small delay to any roundtrip thatuses less than the average roundtrip time. The timeout limit can be setper cursor or per session, for all the cursors as a group. A space limitis set for each cursor's active memory capacity of for all cursors as agroup. Another example of a space limit is for limiting the amount ofpersistent storage occupied by the suspended result set(s). These memoryand/or persistent storage constraints can be set for each cursor, forall cursors in a session, all sessions in a node, and/or all nodes in acluster.

If any constraints have been violated, control returns to the processthat invoked process 240. If the constraints have not been violated,process 240 returns to process action (244) to determine if there aremore rows in the row source to suspend. If there are no more rows in therow source, control returns to the invoking process (such as process 270in FIG. 2D).

Another way to suspend a row of the result set is by the embodiment ofthe suspend a row process 200 shown in FIG. 2A. Process 200 is areentrant process, i.e., it can be stopped and invoked later to suspendadditional rows. As mentioned above, the storage chosen by the suspendprocess can be any of several types. This embodiment shows the use ofpersistent storage as the result set repository. However, the entiresuspended set could be stored in other locations, e.g., in memory ifavailable. Process 200 also uses a write buffer, the row source, and aread buffer. The write buffer is used to hold row bytes of the suspendedresult set being suspended to optimize access to persistent storage, andminimize any performance degradation to the client application. Thewrite buffer receives the result set in sequential order (although theremay be gaps in the sequence) in each call. Gaps may occur because allthe rows need not be suspended. The read buffer is holding a portion ofthe result set for the rows that are being set to the client in order tooptimize access to persistent storage.

The following description of FIG. 2A will reference FIG. 4A, which is arepresentation of the dataflow of the suspending and continuingprocesses. Process action (202) begins the suspend a row process andcreates a Result Set ID.

The Result Set ID serves as a pointer to the suspended result set thatis either passed to the client, stored in a database for laterextraction, and/or used for deleting the suspended result set (andfreeing all its resources) from the database. In one embodiment, theResult Set ID include a logical session identifier for the clientsession and a cursor identifier for the client cursor. The logicalsession identifier can be used to locate the result set when the serverstate of the client session changes (e.g., during transparent sessionmigration). The contents of a Result Set ID need not be interpreted bythe entity that invokes the suspend/continue logic. Further informationregarding session identifiers can be found in co-pending applicationSer. No. 11/071,275, entitled “Global Session Identifiers in aMulti-Node System” filed Mar. 25, 2005, which is hereby incorporated byreference as if fully set forth herein.

In one embodiment, the invoking entity calls the suspend logic with acursor identifier, number of rows to suspend, and a set of constraints.After suspension, the Result Set ID is returned to the invoking entity.The invoking entity passes the Result Set ID to the continue logic alongwith the number of rows to be fetched.

It is determined, in process action (204), if there are more rows in therow source to be stored. One embodiment suspends all the rows of aresult set 402, while another embodiment suspends a selected subset ofthe result set 402. If there are no more rows in result set 402 to bestored, process 200 returns control to the invoking process. If thereare more rows in the row source 402 that need to be suspended, processaction (206) determines if the entire row has been suspended. If theentire row has not been suspended, process action (208) suspends theremaining row bytes from row source 402 to the write buffer 404 andprocess action (210) determines if cursor write buffer 404 is full. Ifthe entire row has been suspended or if the remaining row bytes aresuspended to the write buffer and the writer buffer is not full, processaction (214) determines if the constraint values are violated. Ifprocess action (210) determines that the write buffer is full, the datais flushed into persistent storage 406 in process action (212).

Process action (220) determines if the last fetch obtained data from(points to) the write buffer. If the last fetch points to the writebuffer, process action (222) updates the read buffer 408 from persistentstorage 406. This is done because the full write buffer was just flushedinto persistent storage, therefore, by updating the read buffer frompersistent storage, the data the client is likely to request next isalready in the read buffer. If the fetch does not point to the writebuffer or after the read buffer has been updated, process 200 returns toprocess action (206) to determine if the entire row is suspended and theprocess continues from process action 206 as detailed above.

If process action (214) determines that the constraint values are notexceeded, process 200 returns to process action (204) to determine ifthere are more rows in the row source. If constraints are violated thenprocess action (216) determines if cursor write buffer 404 needs to beflushed to store the remaining data prior to stopping. If the writebuffer needs to be flushed, process action (218) flushes cursor writebuffer 404 into persistent storage 406. Process action (230) determinesif the last fetch obtained data from, or points to, the write buffer. Ifthe last fetch points to the write buffer, process action (232) updatesthe read buffer 408 from persistent storage 406. This is done becausethe full write buffer was just flushed into persistent storage, and byupdating the read buffer from persistent storage, the data the client islikely to request next is already in the read buffer. In process 200, ifthe fetch does not point to the write buffer, or after the read bufferhas been updated, process 200 returns control to the invoking program.The process will return control to the invoking program if a constraintcondition has been exceeded but there is no need to flush the writebuffer as in when the write buffer has already been flushed.

Process 270, the suspend all cursors process is shown in FIG. 2D. It isused for suspending all the cursors in a session. For example, a servermay have idle time and suspend cursor result sets as backgroundoperations. The server would invoke process 270 to accomplish the cursorsuspension. In one embodiment, once the server receives a request fromthe client, it can halt suspension for subsequent rows as a constraintviolation.

Process action (272) starts the suspend all cursors process. Processaction (274) determines if there are more cursors to be suspended. Ifthere are no more cursors to be suspended, process 270 returns controlto the invoking process (e.g., sever background operations). If thereare more cursors to suspend, process action (276) computes theconstraint values for the next cursor. Process action (278) determinesif any constraint values are violated. If the constraint values areviolated, process 270 return control to the invoking process. If theconstraint are not violated, process action (280) invokes the suspend aserver result set process (e.g., process 240, shown in FIG. 2B). Process270 loops back to process action (274) to determine if there are morecursors to suspend.

According to one embodiment, the system can remove or age out one ormore of the suspended result sets. If the client cursor or sessioncorresponding to a result set has ended, then that result set iseligible for removal. Any suitable approach may be used to determinewhich eligible result set should be removed or aged out. For example, aleast recently used approach may be employed to remove one or moreeligible result sets. Alternatively, a clean-up procedure may beemployed to periodically remove some or all of the result sets eligiblefor removal.

During the suspending of the row source, a row mapping index is createdand maintained. This row mapping index, correlates the result set row toa specific storage location and offset. For example, FIG. 2C is arepresentation of one embodiment of such a mapping index. Anotherembodiment has a mapping index for each storage area. In one embodiment,the row mapping index only stores the location index for the next row(e.g., for sequential cursors). In FIG. 2C the result set row number islocated in column 258, and the row storage information is located incolumn 268. For each row number (elements 250-254) in column 258, thereexists a corresponding row storage area (elements 260-264) in column 268that identifies its location. For example, row 1 element 250, is shownin row storage area 260 as located partially in persistent storage atthe persistent storage offset and partially at the read buffer at theread buffer offset. Row 3, element 254, is shown in row storage area 264as located in write buffer at the write buffer offset.

The row mapping index is updated each time any row in the result set ismoved. Some examples of when the row mapping is updated are when therows are suspended into the write buffer, and when the read buffer isupdated with a chunk from persistent storage. The row mapping index canbe stored in one or more of any of the storage mechanisms mentioned inthis disclosure, including as part of the suspended result set.

The process may be optimized with respect to handling of a distributedquery. A distributed query is one in which the query may accessunderlying base data from multiple network locations. In one embodiment,the suspending logic is transparent with respect to the node(s) at whichthe data is located. In an alternate embodiment, suspending a result setrelated to a distributed query may be optimized by first transferringdata responsive to the query from one node to another, and thenperforming the suspend operation at the node which now contains all thedata.

Suspending a result set has many applications. One example applicationof a suspended result set is to be able to continue from the suspendedresult set, which is described in detail below. In one embodiment,suspending and continuing from a suspended result set can be transparentto the client application. In another embodiment, the suspending and thecontinuing operations can be explicitly invoked by the clientapplication. Other examples include: if the system is busy the resultset can be suspended to allow use at a later time and yet free upresources now, if the system wants to retain access to the data afterthe cursor is closed and/or session has ended.

Continuing During Suspending

Continuing during suspending is the term used for the operation ofaccessing a result set while the result set is being suspended. Theclient is allowed to maintain fetch operations while the server issuspending the result set. This provides the client with the desireddata while it allows the server to prepare for the eventual eliminationof the server cursor.

FIG. 1B is a flowchart 100, a generic example of a server processing aclient fetch of a result set with an option of suspending the resultset. The discussion of FIG. 1B will reference FIG. 4B. In process action(101) the client executes a fetch query and the server processes thequery. Process action (102) determines if result set suspension has beenenabled. If result set suspension has not been enabled, the result setis simply fetched from the row source 422 in process action (104).However, if result set suspension has been enabled a Result Set ID iscreated and, process action (106) determines if the precise rowrequested has been suspended, i.e., if the precise row exists in storagelocation 424 used for the suspension process. If the row has beensuspended, process action (108) fetches the row from the suspendedresult set and the fetched row is returned to client 426 in processaction (114). If the row has not been suspended, process action (110)determines if the row should be suspended. If the row should besuspended, process action (112) suspends the row (e.g., into thesuspended result set) (See FIGS. 2A and 2B). If the row is not to besuspended, process action (104) returns the row to client 426 from thecursor row source 422. Process action (116) determines if there are morerows in the result set. If there are more rows in the result set,process 100 returns to process action (102) and repeats the process. Ifthere are no more rows in the result set, process action (118)determines if the cursor should be closed. If the cursor should beclosed, process action (120) closes the cursor. If the cursor is not tobe closed, process action (122) waits for the next client call andreturns to process action (101).

An advantage of the embodiments is that the suspending operation istransparent to the client as the client is still receiving the fetcheddata without noticeable delay. The suspending operation causes minimalimpact on the client fetch operation, so that the client is unaware thatthe suspending operation is taking place. The client is receiving thedata while the server is suspending the result for other uses. Also,note in process 100 that the suspension process can be called at anytime to add more rows to the suspended result set.

FIG. 3 is a flowchart representing another embodiment of a serverprocessing a client fetch of a result set while the result set is beingsuspended, the cursor fetch from a suspended result set process 300. Thefollowing description of FIG. 3 will reference FIG. 4A. Process action(302) begins the fetch row from the suspended result set process.Process action (304) determines if the result set suspension is enabled.If the result set suspension is not enabled, the fetch sends the rowfrom cursor row source 402 to client 410 in process action (306). If theresult set suspension is enabled the Result Set ID is obtained and,process action (305) determines if the specific row has been suspended.

If the row has not been suspended, a determination is made whether therow should be suspended (331). If it is determined that the row shouldbe suspended, then the row is sent from the row source (333) and is thensuspended (335). In another embodiment, one can first suspend the rowand then extract the row starting from action 308 in FIG. 3.Alternatively, and with respect to FIG. 1B, the row is suspended first(112), then fetched (extracted) from the suspended result set (108),then sent to the client (114). One embodiment of suspending a row isdescribed in FIG. 2A. If the row is not to be suspended based upon thedetermination of 331, then the process returns to action 306 to send therow from the row source.

In one embodiment, for sequential cursors, it is not necessary tosuspend rows that are being sent to the client. Therefore, action 331would be not to suspend the row. For scrollable cursors, action 331would be to suspend the row since the client fetches can potentiallyscroll backwards.

If the row has been suspended based upon the action of 305, then processaction (308) determines if the entire row has been sent to the client.If the entire row has been sent to the client, the process returns tothe invoking process.

If the entire row has not been sent to the client, the process action(310) determines if the row bytes are in the read buffer 408. If the rowmapping index indicates that the subject row bytes are located in readbuffer 408, process action (312) locates the row bytes at the indexedoffset in the read buffer and sends the remaining row byes from readbuffer 408 to the client 410. If the row mapping index indicates thatsubject row bytes are not located in read buffer 408, process action(314) determines if the row bytes are located in persistent storage 406(placed there when the write buffer is flushed to persistent storage inthe suspension processes). If the row mapping index indicates thatsubject row bytes are located in persistent storage 406, the row byteoffset from the row mapping index is used to locate the row bytes inpersistent storage and read buffer 408 is updated with a chunk of rowbyte data from persistent storage 406 in process action (316). The rowbytes are sent from read buffer 408 to client 410 in process action(312). If the row mapping index indicates that subject row bytes are notlocated in persistent storage 406, process action (318) determines ifthe subject row bytes are in write buffer 404. If the row mapping indexindicates that subject row bytes are located in write buffer 404,process action (320) locates the row bytes using the offset from the rowmapping index in write buffer 404 and sends the row bytes from writebuffer 404 to client 410. If the row mapping index indicates thatsubject row bytes are not in write buffer 404 (i.e., not in any of thestorage locations), the fetch is in error and is handled by the errorprocessing system.

If the data was sent from the read buffer in process action (312), orthe write buffer in process action (320), process 300 returns to processaction (308) to determine of the entire row was sent to the client or ifmore row bytes are required. If the entire row has not yet been sent tothe client the process precedes to process action (310) as describedabove. If the entire row has been sent to the client, the processreturns to the invoking operation. Using memory buffers as mentionedabove help reduce I/O costs to persistent storage, however, they are nota required element. Any form of storage can be used.

Continuing After Suspending

Continuing after suspending is the term used for the operation ofaccessing a result set that has previously been suspended. One exampleof when continuing from a suspended result set may be required is when aserver cursor is no longer available, perhaps due to the migration ofthe client to another server session. Continuing from the suspendedresult set allows the client cursor access to the result set even thoughthe server cursor is no longer available.

Fetch process 520 shown in FIG. 5A is a generic all purpose example of aserver processing a fetch from a client. Discussion for FIG. 5A willreference FIG. 4B. Process 520 begins with process action (522) afterbeing invoked by client fetch query. Process action (524) determines ifthe fetch is continuing from a suspended result set. If the result setis not suspended, process action (538) sends the row from row source 422to client 426. If the result set is suspended, process action (526)fetches the row from the suspended result set storage 424. Processaction (528) returns the row to client 426. Process action (530)determines if there are more rows to be fetched. If there are more rows,process 520 loops back to process action (524) to process the additionalrows. If there are no more rows in the client query request to befetched, process action (532) determines if the cursor should be closed.If the cursor should be closed, process action (534) closes the cursor.If the cursor should not be closed, process action (536) waits for thenext client call.

Fetch process 500 of FIG. 5B is another embodiment of the cursor fetchprocess after suspending. This process can be used either after thesuspending process is complete or when no result set suspending isnecessary. The discussion for this figure will refer to the FIG. 4Adataflow representation. Note that fetch process 500 does not utilizethe write buffer 404. The data from the write buffer, after suspending,has been stored in the persistent storage 406. In another embodiment,the write buffer 404 can be maintained, for example, in order tooptimize the number of disk I/Os.

Fetch process 500 begins with process action (502). Process action (504)determines if the fetch is continuing from a suspended result set. Ifthe fetch is not continuing from a suspended result set, process action(506) sends the row to the client 410 from the row source 402. If thefetch is continuing from a suspended result set, the Result Set ID isobtained and, process action (508) determines if the entire row has beensent to the client. If the entire row has been sent to the client, theprocess 500 returns control to the invoking operation (e.g., clientquery). If the entire row has not been sent to the client, processaction (510) determines if the subject row bytes are in the cursor readbuffer 408. If the row mapping index indicates that the subject rowbytes are in the read buffer, process action (512) uses the row mappingindex offset to locate the row bytes and sends the remaining row bytesfrom the read buffer 408 to the client 410. Process action (512) loopsback to process action (508) until the entire row has been sent to theclient. If the row mapping index indicates that the subject row bytesare not in the read buffer, then process action (514) determines if thesubject row bytes are located in persistent storage 406. If the rowmapping index indicates that the subject row bytes are located inpersistent storage, process action (516) uses the row mapping indexoffset to locate the row bytes and sends a chunk of data to update theread buffer 408. Process action (512) sends the row bytes from theupdated read buffer 408 to the client 410. If the row mapping indexindicates that the subject row bytes are not located in the cursorpersistent storage 406, the fetch is in error and is handled by theerror processing system.

Note that the movement of result set data (e.g., update of the readbuffer with data from persistent storage) would cause an update to therow mapping index.

Storage compaction can be performed after fetching rows from thesuspended result set, e.g., for sequential cursors. Results sets canpotentially be very large and compaction can therefore minimize theresources that are used to suspend the result set.

In one embodiment, the size of the read buffer and/or write buffer canbe predetermined. In an alternate embodiment, the sizes of the readand/or write buffers can be tuned dynamically, e.g., based upon systemresources. In an embodiment the read buffer could be a read cache withmultiple memory chunks. In another embodiment, the write buffer could bea write cache with multiple memory chunks. The memory chunks need not becontiguous memory.

As described in the earlier sections, there is a read buffer to optimizeaccess to persistent storage, and minimize any performance degradationto the client application. In an embodiment, there need not be a readbuffer or read cache. Continuing fetches from the suspended result setcan be done directly from persistent storage.

Alternatively, there need not be a write buffer or write cache. Thesuspending of the result set can be done directly using the persistentstorage.

In one embodiment, it is not necessary to re-create the server cursorbefore continuing from the suspended result set. In another embodiment,one may re-create the server cursor before continuing from the suspendedresult set.

In one embodiment, the constraint violations are checked after each rowis suspended. In another embodiment the constraint violations may bechecked in the middle of suspending a row, for example, after suspendingeach column.

The suspend and continuing processes described above can be applied tonumerous specific applications. One example application is forscrollable cursors. Another example application of the suspend andcontinuing processes is for using or migrating a result set betweenmultiple client sessions. Another example application is when the serveris busy (e.g., due to server resource constraints or otherwise), theserver may choose to suspend some or all of its cursors in fetch. Oncethe cursors are suspended, the server can release these cursor areasand/or session to reclaim resources. Later when the system is less busyor otherwise, the server can continue fetches from the suspended resultset, in perhaps another (server) session.

For example, consider a result set that is generated for a browsersession which connects to a database server. The browser sessionaccesses the results of a query across one or more web pages through aclient session. Further consider if the browser session is separated orcut off from its associated client session, e.g., because of resourceconstraints or timeout policies. Each client session will have acorresponding session state at the database server. The present approachcan be used to suspend the browser result set when the browser sessionis detached from the client session. When the browser session isreconnected, it may be associated with a different client session.Indeed, the original client session may itself no longer exist. Insteadof re-executing the query, the browser session can directly continuefetching from the suspended result set through a different clientsession.

Cursor Scrolling

One example of continuing a suspended result set is for cursorscrolling. A recent advance in cursor fetching is to allow cursoraccesses to a result set in an arbitrary order. In this approach thecursor can move in any direction or jump anywhere in the result set toaccess a row of data, even for data that has already been fetched. Oneway to fetch data from anywhere in the result set is to convert thecursor request into a row number.

Non-scrollable cursors are limited to fetching rows in forwardsequential manner. According to an embodiment, a scrollable cursor fetchprovides support for forward and backward access to a result set from acurrent position, using absolute or relative row number offsets into theresult set. One may also use keywords such as FIRST, LAST, PRIOR (orPREVIOUS), CURRENT, NEXT to specify the scrollable fetch position.

Note that when a scrollable cursor fetch call, all the rows from thelast row suspended through to all the rows in this client request may besuspended. In another embodiment, the server may suspend more rows thatwhat the client has requested. In another example, if the scrollablecursor requests the last row in the result set, the last row is returnedto the client and all the rows from last row suspended from the rowsource through the last row in the result set are suspended.

In one embodiment, the scrollable cursor fetch is converted into anabsolute row number in the result set. For example, assume that a clientscrollable cursor current position starts at row number 0. Furtherassume the next fetch uses relative positioning with offset +5 and thenumber of rows equals to 10. In this example, the fetch request isconverted to absolute row number current position +5, i.e., 0+5=5. Theserver fetch logic is requested to fetch row numbers 5 through 14. Then,the row mapping index is used to determine the location of the rows inthe suspended result set.

For sequential cursors the fetching is in sequential order. For example,using FIG. 1B, the client executes a query in process action (101).Process action (102) determines if the query result set suspension isenabled. Process 100 fetches the rows from the cursor row source inprocess action (104) without suspending the result set. Morespecifically, as process action (104) calls fetch process 300 in FIG. 3or fetch process 500 in FIG. 5B, process action (306) and (506) bothreturn the result set directly from the row source starting with thefirst row and proceeding sequentially.

For scrollable cursors the client fetch does not have to be in any orderas stated in the example above. That is, the fetching processes, process300 in FIG. 3 and process 500 in FIG. 5B, must be able to return a rowfrom anywhere in the result in any order.

In one embodiment the implementation of scrollable cursors can be todirectly suspend each row it obtains from the row source withoutrequiring an additional scrollable cursor workspace. In anotherembodiment, rows are converted from the scrollable workspace into thesuspended storage. The scrollable workspace format may be different fromthe suspended storage format.

In another embodiment, the scrollable workspace would have the rowsstored in the same format as needed by the suspend/continuing logic. Nowthe copying of rows from scrollable workspace into suspended storage ismore efficient.

In one embodiment, action 204 in FIG. 2A would first suspend the rowsfrom the scrollable workspace and then continue suspending rows from thecursor row source.

In another embodiment, action 204 in FIG. 2A is implemented such thatthe cursor row source itself is scrollable so it would return the row tobe suspended without requiring an additional scrollable workspace. Forsequential cursors the suspending of rows may only be for those rows notyet sent to the client. For scrollable cursors, since the client can askfor any rows(s), including those already seen, the entire result set mayneed to be suspended.

Using FIG. 1B, the client executes a scrollable cursor in process action(101). Process action (102) determines if the result set suspension isenabled. Process action (106) determines if the requested row has beensuspended in the enabled result set suspension process. If the row hasbeen suspended, process 100 initiates a fetch (extract) operation inprocess action (108). That is, process action (108) calls fetch process300 in FIG. 3 (or fetch process 500 in FIG. 5B if the suspending iscomplete). In one embodiment, when a scrollable cursor requests a row ina result set, process action (310) in FIG. 3 or process action 510 inFIG. 5 b (if suspending is complete) determines the absolute row number,uses the row mapping index to find the storage and offset in storage ofthe requested row, updates the read buffer if necessary, and returns therow to the client.

If the row requested has not yet been suspended (as in the case ofjumping ahead with the scrollable cursor), process action (110)determines if the row should be suspended. For a scrollable cursor,process action 110 would determine that the row should be suspended. Fora partial scrollable cursor, it is possible that process action 110would determine not to suspend all rows. If process action (110)determines that the row should not be suspended, a fetch process iscalled to return the row from the cursor row source. If the row shouldbe suspended, process action (112) calls process 200 in FIG. 2A tosuspend the row.

Referring to FIG. 3, in one embodiment, if it is determined to suspend arow (331), then the row is sent from the row source (333) and thensuspended (335). Alternatively, and with reference to FIG. 1B, the rowis suspended first (112), then fetched from the suspended result set(108) and then sent to the client (114).

Transparent Session Migration

One example of suspending a result set and continuing fetches from asuspended result set is for session migration. A session is known as theactive connection between two computers, e.g., between a client and adatabase server. A session is created when an application connects tothe database, and is destroyed when the application logs off. Examplesession information includes client properties such as user identity,user security roles, application name, platform, connection type, etc. Asession can have multiple cursors executing simultaneously, andtherefore, multiple fetches.

One embodiment uses suspended result sets and continuing from suspendedresult sets during transparent session migration in which migration ofserver session state occur in a manner that is transparent to theclient. In an alternate embodiment, the server session migration mayoccur in a manner that is not transparent to the client,. e.g., byallowing the client to control aspects of the server migration, such asthe specific server to which a session will be migrated.

In one embodiment, there can be multiple session migrations. Forexample, a client session can be migrated multiple times, e.g., in acascaded manner. A cascaded migration refers to a sequence in which aclient session migrates multiple times. For example, the client sessionmigrates from a first server session to a second server session, andthen from the second server session to a third server session.

One reason to migrate a session is for load balancing purposes. Forexample, if one processor is heavily loaded while another is not, itwould be advantageous to, increase database throughput, and reduceresponse time to each client, to move one or more sessions from theheavily loaded processor to the processor that is less loaded. However,a session processing a fetch operation cannot be migrated, until thefetch is completed. This causes delays in load balancing operations.

Another reason to migrate an application session is when a serversession becomes unavailable. For example, consider a result set that isgenerated for a browser application session which connects to a databaseserver. The browser session accesses the results of a query across oneor more web pages through a client session. Further consider if thebrowser session is separated or cut off from its associated clientsession, e.g., because of resource constraints or timeout policies. Eachclient session will have a corresponding session state at the databaseserver. The present approach can be used to suspend the browser resultset when the browser session is detached from the client session. Whenthe browser session is reconnected, it may be associated with adifferent client session. Indeed, the original client session may itselfno longer exist. Instead of re-executing the query, the browser sessioncan directly continue fetching from the suspended result set through adifferent client session. It is noted that the browser access can be ofany kind, including either sequential or scrollable.

Transparent session migration may be used for load balancing and dynamicCPU provisioning, and therefore cannot afford to have errors in theresult-set migration. A solution, in one embodiment, is to store theentire pending result set in persistent storage on the server so fetchesfor an application can continue during and after session migration, inexactly the same order with same column values. For sequential cursors,the pending result set includes rows not yet seen by the client. Forscrollable cursors, the pending result set can potentially include theentire result set. As mentioned above, the storage type is dependent onthe application and required use of the result set once suspended. Inanother transparent session migration embodiment, the result set isstored in shared memory or persistent shared memory.

FIG. 6B is a top level flowchart representing a process 600 forexecuting a client call and handling transparent session migration.Process action (602) waits for a client call. Process action (604)processes the client call. If the client call is a fetch, theappropriate fetch process: e.g., fetch during suspending process 300shown in FIG. 3 or fetch after suspending process 500 shown in FIG. 5Bis called. Process 600 determines if the session has been selected formigration in process action (606). Although the flowchart shows thedetermination of migration selection after the client call, the ordercan be reversed. If the session has not been selected for migration theprocess returns to process action (602) and waits for a client call. Ifthe session has been selected for migration, process action (608)invokes the migrate session process 650.

Note that if process actions (606) and (608) are isolated from the restof the diagram, they represent a example of an server execution of aclient call where transparent session migration is not available.

FIG. 6A is a flowchart of a process 650 for suspend and continuing fortransparent session migration according to one embodiment. The followingdescription of FIG. 6A will reference FIG. 7, the data flow diagram forsession migration. In process action (652) the session is selected formigration and process 650 has been invoked. The client 702 is notifiedby the server 714 of the selection in process action (654). Both theserver and the client prepare to migrate in process action (656). During656, the session state information is prepared for migration. Anotheractivity that occurs during migration preparation is suspending thecursor result sets (e.g., cursor 708, and cursor 710) by invokingprocess 270 shown in FIG. 2D. This suspending of all cursors isindependent of the specifics of any client call that may be active.However, the work to suspend is performed after each client call asdescribed above in the SUSPEND section. Process action (658) determinesif the client 702 and the server 714 are prepared to migrate the session716. If the client and the server are not prepared to migrate, flowreturns to process action (656) to continue migration preparations. Ifthe client 702 and the server 714 are prepared to migrate session 714,process action (660) opens a new session 726 on server 724, copies thesession state information into the new session 726, and creates aconnection from the client 702 to new session 726 on server 724. Theclient 702 makes calls to the new session 726 in process action (662)and the result set stored in persistent storage 706 of database 704 isreturned to the client 702 in process action (664).

FIG. 7 is a representation of a client continuing a suspended resultset. Persistent storage 706 is shareable across nodes and is used as oneexample, but other storage types may be used without detraction from theembodiment such as shared memory 722. Client 702 has an 2 client sidecursors 708 and 710 communicating with server cursor 718 and 720 viaconnection 719 to server session 716 of Server 714. Client cursors 708and 710 are fetching result set data from server cursors 718 and 720respectively using fetching process 300, shown in FIG. 3. The resultsets of these cursors are suspended in database 704 in shared persistentstorage 706 using the suspend process 200 shown in FIG. 2A, and theclient 702 is unaware of the activity of the suspend operation. Oncesession 726 is available on server 724, client 702 connects to it viaconnection 721 to access the cursor result sets. Server 724 can accessthe suspended result set from shared persistent storage 706. In anotherembodiment, server 724 can access the suspended result set from sharedmemory 722. In another embodiment, server 724 car receive a copy of thesuspended result set for cursor 718 and 720 to be stored on server 724,in suspended result set 728 and 730, respectively. Once the connection721 to the new server session 726 is established, connection 719 to theoriginal server 714 is severed.

In one embodiment, once the entire result set is suspended, client 702and server 714 synchronize a session migration. In another embodiment,the entire result set does not have to be suspended to proceed withsession migration. Only the rows which the client can potentially fetchneed to be suspended to proceed with session migration. The secondsession, session 726 on server 724 is opened and made available toclient 702. Session 726 has access to persistent storage 710 and canhonor any data request for data in the result set. The data requestsfrom client 702 to session 726 are not satisfied from the originalserver cursor execution area. Another embodiment allows for cascadedsession migration.

Client 732 is another client that can now access the result sets ofcursors 718 and 720 since they are located in persistent storage 710. Inone embodiment, Client 732 accesses the persistent storage using datafile requests. In another embodiment, the persistent storage data filemay be transferred to client 732 as one file allowing client 732 toaccess the result set on its own station and not have to use networkresources. In another embodiment, client 732 uses SQL or programmaticfetch APIs to access the suspended result set.

System Architecture Overview

The execution of the sequences of instructions required may be performedin some embodiments by a computer system 1400 as shown in FIG. 8. In anembodiment, execution of the sequences of instructions required isperformed by a single computer system 1400. According to otherembodiments, two or more computer systems 1400 coupled by acommunication link 1415 may perform the sequence of instructionsrequired in coordination with one another. In order to avoid needlesslyobscuring the explanation, a description of only one computer system1400 will be presented below; however, it should be understood that anynumber of computer systems 1400 may be employed.

A computer system 1400 according to an embodiment will now be describedwith reference to FIG. 8, which is a block diagram of the functionalcomponents of a computer system 1400 according to an embodiment. As usedherein, the term computer system 1400 is broadly used to describe anycomputing device that can store and independently run one or moreprograms.

Each computer system 1400 may include a communication interface 1414coupled to the bus 1406. The communication interface 1414 providestwo-way communication between computer systems 1400. The communicationinterface 1414 of a respective computer system 1400 transmits andreceives electrical, electromagnetic or optical signals, that includedata streams representing various types of signal information, e.g.,instructions, messages and data. A communication link 1415 links onecomputer system 1400 with another computer system 1400. For example, thecommunication link 1415 may be a LAN, in which case the communicationinterface 1414 may be a LAN card, or the communication link 1415 may bea PSTN, in which case the communication interface 1414 may be anintegrated services digital network (ISDN) card or a modem, or thecommunication link 1415 may be the Internet, in which case thecommunicate interface 1414 may be a wireless, cable, or dial-up modem.

A computer system 1400 may transmit and receive messages, data, andinstructions, including program, i.e., application, code, through itsrespective communication link 1415 and communication interface 1414.Received program code may be executed by the respective processor(s)1407 as it is received, and/or stored in the storage device 1410, orother associated non-volatile media, for later execution.

In an embodiment, the computer system 1400 operates in conjunction witha data storage system 1431, e.g., a data storage system 1431 thatcontains a database 1432 that is readily accessible by the computersystem 1400. The computer system 1400 communicates with the data storagesystem 1431 through a data interface 1433. A data interface 1433, whichis coupled to the bus 1406, transmits and receives electrical,electromagnetic or optical signals, that include data streamsrepresenting various types of signal information, e.g., instructions,messages and data. In embodiments, the functions of the data interface1433 may be performed by the communication interface 1414.

Computer system 1400 includes a bus 1406 or other communicationmechanism for communicating instructions, messages and data,collectively, information, and one or more processors 1407 coupled withthe bus 1406 for processing information. Computer system 1400 alsoincludes a main memory 1408, such as a random access memory (RAM) orother dynamic storage device, coupled to the bus 1406 for storingdynamic data and instructions to be executed by the processor(s) 1407.The main memory 1408 also may be used for storing temporary data, i.e.,variables, or other intermediate information during execution ofinstructions by the processor(s) 1407.

The computer system 1400 may further include a read only memory (ROM)1409 or other static storage device coupled to the bus 1406 for storingstatic data and instructions for the processor(s) 1407. A storage device1410, such as a magnetic disk or optical disk, may also be provided andcoupled to the bus 1406 for storing data and instructions for theprocessor(s) 1407.

A computer system 1400 may be coupled via the bus 1406 to a displaydevice 1411, such as, but not limited to, a cathode ray tube (CRT), fordisplaying information to a user. An input device 1412, e.g.,alphanumeric and other keys or a pointing device such as a mouse, iscoupled to the bus 1406 for communicating information and commandselections to the processor(s) 1407.

According to one embodiment, an individual computer system 1400 performsspecific operations by their respective processor(s) 1407 executing oneor more sequences of one or more instructions contained in the mainmemory 1408. Such instructions may be read into the main memory 1408from another computer-usable medium, such as the ROM 1409 or the storagedevice 1410. Execution of the sequences of instructions contained in themain memory 1408 causes the processor(s) 1407 to perform the processesdescribed herein. In alternative embodiments, hard-wired circuitry maybe used in place of or in combination with software instructions. Thus,embodiments are not limited to any specific combination of hardwarecircuitry and/or software. In one embodiment, the term “logic” shallmean any combination of software or hardware that is used to implementsome or all of the embodiments.

The term “computer-usable medium,” as used herein, refers to any mediumthat provides information or is usable by the processor(s) 1407. Such amedium may take many forms, including, but not limited to, non-volatile,volatile and transmission media. Non-volatile media, i.e., media thatcan retain information in the absence of power, includes the ROM 1409,CD ROM, magnetic tape, and magnetic discs. Volatile media, i.e., mediathat can not retain information in the absence of power, includes themain memory 1408. Transmission media includes coaxial cables, copperwire and fiber optics, including the wires that comprise the bus 1406.Transmission media can also take the form of carrier waves; i.e.,electromagnetic waves that can be modulated, as in frequency, amplitudeor phase, to transmit information signals. Additionally, transmissionmedia can take the form of acoustic or light waves, such as thosegenerated during radio wave and infrared data communications.

In the foregoing specification, the embodiments have been described withreferences to specific elements. However, it will be evident thatvarious modifications can changes my be made thereto without departingfrom the broader spirit and scope. For example, the reader is tounderstand that the specific ordering and combination of process actionsshown in the process flow diagrams herein is merely illustrative, andthe embodiments can be performed using different or additional processactions, or a different combination or ordering of process actions. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than restrictive sense.

1. A computer-implemented method for suspending a cursor result set fortransparently migrating a first session, the method comprising:receiving notification the first session at a first server is selectedfor migration; preparing to migrate, preparing including suspending acursor result set for the first session; suspending the cursor resultset by storing the cursor result set in a storage area of acomputer-usable medium for later access without re-execution of a query;determining if the first session is ready to migrate; and migrating thefirst session at the first server to a second server, the migratingcomprising: opening a second session on the second server; copyingsession state of the first session into the second session; and creatinga connection to the second session on the second server, wherein theconnection to the second session on the second server allows forcontinued execution of the query from the suspended cursor result setwithout re-execution of the query.
 2. The method of claim 1 whereinmigrating comprises: opening the second session; copying the suspendedcursor result set to the second session; connecting a client to thesecond session; and breaking the client connection to the first session.3. The method of claim 2, further comprising executing a fetch requeston the suspended cursor result set from the second session.
 4. Themethod of claim 3, wherein executing the fetch request on the suspendedcursor result set comprises: determining a location of a subject row ofthe cursor result set; and returning the subject row from the location.5. The method of claim 2, wherein executing a call on the suspendedresult set via the second session is preceded by the same cursorexecuting a call on the result set via the first session.
 6. The methodof claim 5, wherein the storage area comprises memory, persistentstorage, or a combination of persistent storage and memory.
 7. Themethod of claim 5, wherein the storage area includes all the rows incursor result set.
 8. The method of claim 2, wherein the client isunaware that a session is migrating or of a switch from the firstsession to the second session.
 9. The method of claim 1, whereinsuspending cursor result sets comprises storing the rows of each cursor,each cursor's rows stored into the storage area associated with acorresponding cursor.
 10. The method of claim 1 in which a clientaccesses session data from the first session before migration hascompleted.
 11. The method of claim 1 in which a client accesses sessiondata from the first session after migration has completed.
 12. Acomputer-implemented method for suspending a cursor result set andcontinuing a suspended cursor result set for transparent sessionmigration, the method comprising: executing a query on a first sessionon a first server; receiving notification the first session was selectedfor migration; preparing to migrate the first session; suspending acursor result set by storing the cursor result set in a computer-usablestorage medium for later access without re-execution of a query;determining if ready to migrate the first session; and migrating thefirst session at the first server to a second server and accessing thecursor result set from the suspended cursor result set, the migratingcomprising: opening a second session on the second server; copyingsession state of the first session into the second session; and creatinga connection to the second session on the second server, wherein theconnection to the second session on the second server allows forcontinued execution of the query from the suspended cursor result setwithout re-execution of the query.
 13. The method of claim 12, whereinpreparing to migrate comprises: determining the cursor result set isbeing fetched; computing a set of constraint values for a cursor; andsuspending the result set of the cursor.
 14. The method of claim 13,wherein preparing to migrate further comprising sending notification tothe client of the selection for migration.
 15. The method of claim 13,the suspending further comprising: a) caching a row into a storagelocation; b) determining if any of the set of constraint values havebeen exceeded; c) determining if there are more rows to be suspended ifthe constraint values have not been exceeded; and d) repeating a)-c) ifthere are more rows to be suspended.
 16. The method of claim 12, whereinmigrating comprises: opening the second session on a second server;copying the first session's state information to the second session;connecting a client to the second session; and breaking connectionbetween the client and the first session.
 17. The method of claim 16,further comprising executing a call on the suspended result set data viaa second session.
 18. The method of claim 17, wherein the continuing thesuspended cursor result set further comprises: e) determining that theresult set is suspended; f) determining the location of a subject row ofthe result set; g) repeating e)-g) until all the required rows have beenfetched.
 19. The method of claim 17, wherein executing a call on thesuspended result set via a second session is preceded by a cursorexecuting a call on the result set via the first session.
 20. The methodof claim 16, wherein suspending a result set comprises: determining thecursor result set is to be suspended; storing a first row of the cursorresult set in a storage area; and repeating the storing until all therequired rows have been suspended.
 21. The method of claim 20, whereinthe suspending is performed in a manner so that the performancedegradation of the returning rows to the client caused by the suspendingis unnoticeable to a user.
 22. The method of claim 12, wherein a user isunaware of switching from the first session to the second session. 23.The method of claim 12, wherein the suspended result set is transmittedfrom one node to another node.
 24. The method of claim 12, wherein theexecuting query is a distributed sequential query language (SQL) query.25. The method of claim 12 wherein the query does not have to adjust anyapplication parameters due to the migration.
 26. The method of claim 12,wherein the transparent session migration is a cascaded transparentsession migration.
 27. A computer program product comprising a computerusable storage medium having executable code to execute a process forsuspending a cursor result set and continuing a suspended cursor resultset for transparent session migration, the process comprising: executinga query on a first session on a first server; receiving notification thesession was selected for migration; preparing to migrate, preparingincluding suspending a cursor result set for the first session;suspending the cursor result set by storing the cursor result set forlater access without re-execution of a query; determining if ready tomigrate the first session; and migrating the first session at the firstserver to a second server, when ready and accessing the cursor resultset from the suspended cursor result set, the migrating comprising:opening a second session on the second server; copying session state ofthe first session into the second session; and creating a connection tothe second session on the second server, wherein the connection to thesecond session on the second server allows for continued execution ofthe query from the suspended cursor result set without re-execution ofthe query.
 28. The computer program product of 27, the process furthercomprising: executing a fetch request on the suspended cursor resultsets from the second session.
 29. The computer program product of 27,wherein a client is unaware that a session is migrating or of a switchfrom the first session to the second session.
 30. A system comprising acomputer usable storage medium for suspending a cursor result set andcontinuing a suspended cursor result set for transparent sessionmigration, comprising: means for executing a query on a first session ona first server; means for receiving notification the first session wasselected for migration; means for preparing to migrate the firstsession; means for suspending a cursor result set by storing in thecomputer usable storage medium the cursor result set for later accesswithout re-execution of a query; means for determining if ready tomigrate the first session; and means for migrating the first session atthe first server to a second server and accessing the cursor result setfrom the suspended cursor result set, the migrating comprising: openinga second session on the second server; copying session state of thefirst session into the second session; and creating a connection to thesecond session on the second server, wherein the connection to thesecond session on the second server allows for continued execution ofthe query from the suspended cursor result set without re-execution ofthe query.
 31. The system of claim 30, further comprising: means forexecuting a fetch request on the suspended cursor result sets from asecond session.
 32. The system of claim 30, wherein a client is unawarethat a session is migrating or of a switch from the first session to thesecond session.
 33. A computer program product comprising a computerusable storage medium having executable code to execute a process forsuspending a cursor result set for transparently migrating a firstsession, the process comprising: executing a query on a first session ona first server; receiving notification the first session was selectedfor migration; preparing to migrate the first session; suspending acursor result set by storing the cursor result set for later accesswithout re-execution of a query; determining if ready to migrate thefirst session; and migrating the first session at the first server to asecond server, the migrating comprising: opening a second session on thesecond server; copying session state of the first session into thesecond session; and creating a connection to the second session on thesecond server, wherein the connection to the second session on thesecond server allows for continued execution of the query from thesuspended cursor result set without re-execution of the query.
 34. Thecomputer program product of 33, the process further comprising:executing a fetch request on a suspended cursor result sets from thesecond session.
 35. The computer program product of 33, wherein a clientis unaware that a session is migrating or of a switch from the firstsession to the second session.
 36. A system comprising a computer usablestorage medium for suspending a cursor result set for transparentlymigrating a first session, the method comprising: means for executing aquery on a first session on a first server; means for receivingnotification the first session is selected for migration; means forpreparing to migrate, preparing including suspending a cursor result setfor the first session; means for suspending the cursor result set bystoring in the computer usable storage medium the cursor result set forlater access without re-execution of a query; means for determining ifthe first session is ready to migrate; and means for migrating the firstsession at the first server to a second server, the migratingcomprising: opening a second session on the second server; copyingsession state of the first session into the second session; and creatinga connection to the second session on the second server, wherein theconnection to the second session on the second server allows forcontinued execution of the query from the suspended cursor result setwithout re-execution of the query.
 37. The system of claim 36, furthercomprising: means for executing a fetch request on a suspended cursorresult sets from the second session.
 38. The system of claim 36, whereina client is unaware that a session is migrating or of a switch from thefirst session to the second session.